Sample-Return: Space Explorers send back more than just PicturesFollow article
Hayabusa-2 about to touchdown briefly on asteroid Ryugu and collect a sample. It didn’t land; instead, it fired a ‘bullet’ into the surface which kicked up fragments of rock and dust. These were collected in the sample horn visible underneath as it moved away. - Image credit: JAXA/EPA
The Japanese spacecraft Hayabusa-2 last week sent back to Earth 5.4g of material taken from the near-Earth asteroid Ryugu. The first rock samples from another ‘world’, were brought back by the Apollo 11 astronauts returning in triumph from the Moon in 1969. Since the end of the Apollo project, scientists wishing to study such physical objects have had to rely on robotic explorers like Hayabusa-2 to provide them. The old USSR was very keen on robotic sample-returners, but with patchy results and many failures. Both NASA and ESA have generally taken the approach of incorporating a robotic science lab into the spacecraft, in theory eliminating the need for a return to Earth. More robotic explorers of both types are scheduled to sample bits of comets, asteroids and planets in the future.
Observing the Universe
Until Galileo realised in 1609 that the newly-invented telescope was ideal for getting a better view of those white dots in the sky, astronomers could only study and characterise their movement, not their composition. Of course, the telescope was not a new sensor – just a signal amplifier for the human eye. As well as being able to see in (reasonable) detail the rings of Saturn and the coloured bands of Jupiter, later scientists were able to apply a new analysis technique to the telescopic images: Spectrometry. By analysing the spectrum of light coming from a star, it’s possible to determine which chemical elements are present. The same analysis can be performed on sunlight reflected from our nearby planets. This sort of work is still carried out today using enormous optical instruments situated in the clean, rarefied atmosphere of mountain tops. But to study the light from planets around those distant stars, instruments need to be located above the Earth’s interfering atmosphere and the sensitivity extended beyond visible light into the InfraRed and Ultraviolet wavelengths. Hence a series of robotic telescope spacecraft starting with Hubble were flown.
A Closer Look
It’s amazing what information can be gleaned from a few photons of light reaching a detector: a lot can be learned about the atmosphere of a planet orbiting a distant star by spectroscopic analysis of that star’s light shining through it. This is all very clever, and maybe it’s all we can get from objects hundreds of light-years away. But what about planets and other rocky objects in our own solar system, and who cares anyway?
A Bit of History
During the 1960s the answer to the second part of that question would have been practically nobody, apart from a few astronomers. The real interest lay with launch vehicles and spacecraft for looking down at the Earth, not outward to the solar system. In other words, Intercontinental Ballistic Missiles (ICBM) and spy satellites for use in the Cold War between the USA and USSR. The latter started the Space Race in 1957 with the orbiting of the world’s first artificial satellite, Sputnik 1. It was a clever move on the part of the Russians because it demonstrated the capability of their R-7 Semyorka ICBM without actually using it! It had the desired effect and panic ensued in the US which thus far, had been a bit lukewarm on Space technology. After that, Russian technological triumphs such as sending the first man into orbit were interspersed with embarrassing US failures or catch-ups. That is, until President John F. Kennedy announced the NASA Apollo project to send a man to the Moon before the year 1970.
Sample-Return with Astronaut Assistance
The motivation for Apollo was undoubtedly political, but scientific and engineering knowledge benefited enormously. These were the first ever sample-return missions from a body outside the Earth’s atmosphere – in this case the Moon. Because a rather large spacecraft was used, able to accommodate three astronauts, the returns were scientifically more useful than if a robot had been sent:
- Each successive mission (apart from Apollo 13 of course) brought ever greater quantities of material, starting with Apollo 11’s modest 22kg to 17’s massive 111kg - a total of 382kg.
- The returned ‘sample’ actually consisted of many individual samples from drill cores and scoops of lunar regolith to individual rocks.
- Relatively large areas were explored, especially when the lunar rover became available on the last three missions, 15 to 17.
- The astronauts received training in lunar geology and while working with scientists back at mission control were able to be very selective in their choice of ‘moon rocks’ to bring back to Earth. The last mission, Apollo 17 had the additional benefit of a fully-trained science-astronaut, Harrison Schmitt, further improving the selection process.
- Long-term automatic sensing packages, e.g. a seismograph, were laid out on the surface, some still working years after the astronauts had left.
Since the Apollo 17 mission in 1972, all sample-gatherers have been robotic and until recently, not very intelligent, with all that implies.
Sample-Return by Robotic Lander
As a consolation prize after the Americans beat them to the Moon with Apollo 11 in 1969, the USSR did mount the first successful robotic lunar sample-return mission in 1970, Luna 16, which gave them a 101gm sample. This was followed by Luna 20 (55gm) and Luna 24 (170gm). Not a lot to show for three successful missions and eight failures. The missions failed for a variety of reasons, but the main one common to all is that Space technology, especially robotic was in its very early development phase.
USSR Luna 16 Lander 1970 - Image credit: Wikipedia
Even the Americans lost the first six of their Ranger lunar probes between 1961 and 1964. The Rangers can’t be described as landers: more like deliberate crash-landers; all they had to do was send back lots of digitised pictures of the surface, along with some other measurements before crashing into the Moon. Finally, the last three Rangers delivered the pictures desperately needed by NASA to assess landing sites for the upcoming Apollo missions. A possible reason for some of the losses was found to be an inability of a semiconductor diode to survive the harsh Space environment.
After Ranger came Surveyor, a series of seven genuine landers launched between 1966 and 1968 to test the feasibility of er… landing on the Moon! Five of them landed successfully. Data on the consistency of the lunar regolith was urgently needed to prepare for the manned Apollo landings. At the time some scientists believed that the regolith would be too soft and deep to support the weight of a LEM. Fortunately, Surveyor proved otherwise. They were simple machines and although they featured a sample scoop, it was used for basic mechanical tests – nothing chemical or biological. In other words, they were not capable of sample-return and had no on-board automated laboratory instruments: unlike the next generation of landers.
CNSA Cheng’e-5 Lunar Lander 2020 - Image Credit: CNSA/CLEP
This week, the first lunar sample-return mission since 1976 has been successfully completed by the Chinese lander Cheng’e-5. The lander looks remarkably like an Apollo LEM and just like Apollo, its ascent stage had to dock with an orbiter for the journey home. Cheng’e-5 is a lot more sophisticated than the Soviet Luna probes of the 1970s though, as it carried out a variety of measurements and returned about 1.7kg of drill-cores and scoop samples.
Sample-Analysis by Robotic Lander (Robotic Lab)
In 1975 two landers plus orbiters were sent to Mars. Called Viking, each was similar in appearance to Surveyor, but included a robotic science laboratory with experiments to detect signs of any microbial life present in soil samples. This type of lander only returns data to Earth. Initial analysis of results from the experiments did seem to confirm some form of biological life in the Martian soil, but this conclusion was later disputed. Until recently, Mars has proved to be something of a graveyard for both landers and orbiters: Soviet missions usually ending in failure for one reason or another. Even NASA has lost two: Mars Climate Orbiter and Mars Polar Lander both caused by trivial undetected design errors generally attributed to a cost-cutting initiative in operation at the time.
The UK’s Beagle 2 Mars lander almost made it, but for a solar panel failing to deploy after touchdown. In this case, the very low budget available for this attempt to detect life on Mars meant that design compromises ultimately made the project unlikely to succeed. For years its whereabouts were unknown after disappearing as it entered the Martian atmosphere in December 2003. In 2014 something was spotted on the surface by the Mars Reconnaissance Orbiter and identified as the lost Beagle 2 lander.
NASA InSight Mars Lander 2018 is designed to look deep into the planet’s structure – not just scratch the surface. Seismometer on the left, heat-flow ‘mole’ on the right. - Image credit: NASA
Recent NASA Mars landers have been more successful: Phoenix in 2008 and the very similar design InSight in 2018. The latter’s payload differs somewhat from most other ‘traditional’ landers in that the surface is not sampled, returned or analysed. Instead, a very sensitive seismograph was placed on the ground by a robot arm and a temperature probe has attempted to burrow beneath the surface to a depth of 5 metres. There is also a comprehensive weather station on board which, in addition to its data-gathering role, allows scientists to filter out vibrations detected by the seismometer caused by the Martian wind. Unfortunately, the soil structure may defeat the ‘mole’, described as a self-hammering nail: it’s struggling to get anywhere near 5m down at the time of writing. These sensors will, hopefully, provide a greater understanding of the Martian core, not just a bit of the surface.
Sample-Return by Mobile Robots (Rovers)
The trouble with static landers is that they can’t move around if the bit of geology they want to look at is just out of reach of their sampling tool. It seems easy to solve the mobility problem by adding wheels, motors and a radio-control system as used by military drones. For direct ‘real-time’ control, the limiting factor is the distance between the controller and the rover. Radio waves travel at the speed of light, but still take between 5 and 20 minutes to reach Mars, depending on where it is in its orbit relative to Earth. No direct control for Mars then, but the 2.5 seconds delay to the Moon is manageable. In 1970 the Soviet robotic rover Lunokhod 1 charged around the Moon under near real-time control of operators on Earth. It was able to take measurements of radiation and test the lunar regolith, but its primary objective was to set records and score political points in the Cold War. The Apollo lunar rover was essentially a very expensive electric car driven by astronauts!
Starting with Pathfinder-Sojourner in 1997 up to Curiosity in 2012 there has been an almost unbroken line of NASA robotic rovers on Mars. Another, Perseverance is on its way and scheduled to arrive in February 2021. Superficially similar-looking to Curiosity, this rover will have a very special task to perform: collect rock core samples and seal them in tubes for return to Earth. The following NASA video shows how this collection and storage is achieved in superb detail:
A future mission will see another rover arrive to collect these sample tubes, take them back to a lander where an ascent rocket will shoot them to a return vehicle in orbit which will finally bring them back to Earth.
NASA/ESA Mars Sample Return ascent rocket takes off (proposal). The Perseverance rover on its way to Mars now will collect samples in special tubes. The design for the rest of the return mission has yet to be finalised. - Image Credit: NASA
Design considerations for Sample-Return missions
Apart from a million other things to think about, there are two fundamental characteristics of the target celestial body that are major design drivers:
- The presence or otherwise of an atmosphere.
- The body’s mass and hence its force of gravity.
The presence of an atmosphere, even if it’s as thin as the one around Mars, will require the lander to have a heatshield to protect it on the way down. On the plus side, aerobraking will be possible and a parachute may be deployed to slow the spacecraft down for landing. The Moon has no atmosphere so Luna 16 had no heatshield, but it did need a rocket engine to slow its descent to the surface. The ascent stage required no streamlined panels either – no air = no aerodynamics. On the hand, you can see from the picture of the proposed Mars Return system: the ascent stage is a streamlined rocket.
The lower gravity on the Moon and Mars has been of great benefit to the sample-return missions. It means that ascent engines can be a lot less powerful than they would need to be on Earth. Unfortunately, if the target body is really small with very little gravity, it makes it very difficult to land and more importantly, stay landed. The Philae lander was carried to comet 67P as part of ESA’s Rosetta mission. An upward thruster designed to fire as Philae landed failed to do so (It had been in transit for 10 years). This failure allowed the lander to ‘bounce’ before anchors could deploy and it ended up in the shadow of a cliff with no sunlight to recharge its batteries.
Landing on an Asteroid
Lessons were learned from Rosetta, and neither Hayabusa-2 which has just returned to Earth from asteroid Ryugu nor NASA’s OSIRIS-REx now on its way back from asteroid Bennu actually landed. In each case, an extended collector probe beneath the lander touched down briefly causing Hayabusa to fire a bullet into the surface, OSIRIS-REx a blast of nitrogen gas. The resulting hailstorm of dust and rocks was thus blown into a collection container. The reaction to the blast acted to lift the landers away from the asteroid immediately after touchdown and sample collection.
Human versus Machine
Is it necessary for astronauts to risk their lives going to the Moon and Mars when robots can do the job of exploring Space? It is arguable that current robot technology, even with the Artificial Intelligence features of Perseverance and the yet to launched Rosalind Franklin, cannot match the dexterity of the human body or the analysing/decision-making capability of the brain. Consider one definition of the word ‘Exploration’: ‘the action of exploring an unfamiliar area’. The key word is unfamiliar. Machines are very good at following a list of instructions – a program – where all possible actions have been thought out in advance, but are lousy at handling situations that have not been covered. So, almost by definition they are incapable of exploring ‘the unknown’. There are ways of mitigating this problem, the most obvious being to use ‘cheap’ simple probes as pathfinders such as Surveyor prior to the Apollo landings. Sample-gathering missions such as Hayabusa-2 and OSIRIS-REx may be precursors for asteroid mining operations in the future. I believe it’s inevitable that astronauts will return to the Moon and then move on to Mars: the human need to explore the unknown is just too intense for it to be satisfied by robots alone. Mars rovers Perseverance and Tianwen-1 land in 2021 while Rosalind Franklin will have to wait for the next launch window in September 2022 to start its journey. They will form the next part of the Mars pathfinder force, helping to ensure the safe return to Earth of the first astronauts to visit another planet, along with all their rock samples and new-found knowledge.
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